1. Field of the Invention
This invention concerns a reflective layer or semi-transparent reflective layer for use in optical information recording media (reflective layer for optical disc or semi-transparent reflective layer for optical disc) in which the diffusion of Ag is suppressed and crystal grain growth is suppressed, optical information recording media and a sputtering target for use in the reflective layer of the optical information recording media.
Since the reflective layer or semi-transparent reflective layer of this invention has a high reflectivity, it is used suitably, for example, to read-only optical discs (writing-rewriting impossible) such as CD-ROM, DVD-ROM; a writing-once type optical disc (recording only for once and repeated reading possible) such as CD-R or DVD-R; and writable type optical disc (capable of repeating recording/reading) such as CD-RW, DVD-RAM, DVD-RW, DVD+RW and PD.
Optical discs include several types and they are generally classified into three types of {circle around (1)} read-only optical disc, {circle around (2)} writable optical disc and {circle around (3)} writing-once optical disc in view of the writing/reading principle.
Among them, the read-only optical disc {circle around (1)} basically comprises a reflective layer using Ag, Al, Au and so on as a base metal and a protective layer such as made of a UV curable resin protective layer stacked on a transparent plastic substrate such as a polycarbonate substrate. The read-only optical disc is adapted to form recording data by concave/convex pits formed on a transparent plastic substrate and read data by detecting the phase difference or reflection difference of a laser beam irradiated on the disc. In addition to the stack type described above, there is also used a disc as shown in FIG. 1 in which a base material provided with a semi-transparent reflective layer 2 and a base material provided with a reflective layer 4 are appended by way of an adhesion layer 3 on a transparent plastic substrate 1 and, further, a transparent plastic substrate 5 is laminated thereon. Optical disc adopting such data read-only (writing-recording impossible) system can include, for example, CD-ROM, DVD-ROM.
Then, the writable type (phase change type) optical disc {circle around (2)} above is adapted to record data by forming a 2-phase state, that is a crystalline phase and an amorphous phase on a recording layer by controlling the power and the irradiation time of a laser beam and read the data by detecting the change of the reflectivity of both phases by a laser. In this recording/reading system, repeating recording/reading is possible and recording can be conducted repeatedly usually by about several thousands to several hundreds thousands of times. The basic structure of the writable optical disc comprises, as shown in FIG. 2, various thin film layers, i.e., a dielectric layer 7, a recording layer 8, a dielectric layer 7, a reflective layer 4 and a transparent plastic substrate 5 are laminated on a transparent plastic substrate 1. The optical disc adopting such a system can include, for example, CD-RW, DVD-RAM, DVD-RW, DVD+RW.
Further, the writing-once type optical disc {circle around (3)} is adapted to record data by heating to denature a dye of a recording thin film layer (organic dye layer) by a power of laser beam thereby deforming grooves (grooves previously formed on a substrate) and read the data by detecting the difference between the reflectivity at a denatured zone and the reflectivity at a not denatured zone. FIG. 3 shows a basic structure of the writing-once type optical disc. In the figure, are shown a transparent plastic substrate 1, an organic dye layer 6, a reflective layer 4 and a transparent plastic substrate 5. The recording/reading system has a feature in that once recorded data can not be rewritten (recording only for once and repeating reading) and the optical disc using such a system can include, for example, CD-R and DVD-R.
In each of the optical discs described above, it is required for the material of the reflective layer to have excellent properties such as reflectivity, thermal conductivity, thermal shock resistivity, and chemical stability (particularly, corrosion resistivity (oxidation resistivity)) and cause less aging deterioration in the recording/reading characteristics.
For example, since the reflective layer for use in the writable optical disc {circle around (2)} also serves as a heat diffusion layer, it is further required to be excellent in the thermal conductivity in addition to the characteristics described above. Particularly, in high density recording, high thermal conductivity of the heat dissipation layer is essential in view of the improvement for the recording density. However, materials for use in the reflective layer capable of satisfying such required characteristics has not yet been provided at present.
For example, Al alloys used generally as the reflective layer for use in the writable optical discs have relatively high reflectivity to the wavelength of laser used for recording/reading (780 nm, 650 nm) and corrosion resistivity (chemical corrosion resistivity) but they are not yet sufficient in view of the reflectivity and have a drawback in that the reflectivity is poor in comparison with that of the Au or Ag system. Further, when compared with the Au system, they also involve drawbacks that the chemical stability is poor and the thermal conductivity is low. Particularly, they also involve a drawback that they are poor in view of the high thermal conductivity required for each disc of writable type and writing once type discs. Accordingly, when Al alloys are used for the reflective layer, it is difficult to provide the reflective layer with required characteristics, to result in a disadvantage of restricting the structure and the design of the disc.
In view of the above, while use of Au, Ag and Cu as the material for the reflective layer instead of the Al alloys has been proposed, each of them has the following problems.
For example, pure Au or alloy comprising Au as the main ingredient is advantageous in excellent chemical stability, causing less aging deterioration for recording/reading characteristics and capable of providing high reflectivity, high corrosion resistivity and high thermal conductivity, but Au is extremely expensive and is not practical. Further, it has a problem that no sufficient reflectivity can be obtained for laser with lower wavelength (ex. 405 nm) applied for further higher recording density disc.
Further, while Cu or alloy comprising Cu as the main ingredient is inexpensive, this has drawbacks of poor corrosion resistivity (particularly oxidation resistivity) and low reflectivity to the laser with lower wavelength like that in the Au series. As a result, Au-based reflective layer can not be applied for higher density recording media.
Further, while pure Ag or alloy comprising Ag as the main ingredient shows sufficiently excellent high reflectivity at a practical wavelength region of 400 to 800 nm, this has a drawback of being inferior to the Au series reflective layer in view of the corrosion resistivity and the aging deterioration for the recording/reading characteristics. Particularly, the Ag-based alloy involve a problem that since Ag diffuses easily, structural change such as crystal growth occurs easily to deteriorate the film property.
On the other hand, the reflective layer of the writing-once type optical disc {circle around (3)} also causes problems similar with those of the writable optical disc {circle around (2)} described above.
In the writing-once type optical disc, Au or alloy comprising Au as the main ingredient is generally used as the material for use in the reflective layer. Such material can attain a high reflectivity of 70% or more to the laser wavelength used for recording/reading (780 nm, 650 nm). However, as described above, Au is extremely expensive, which is mainly attributable to the increase of cost.
In view of the above, it has been proposed to use Ag, Cu or Al as the material for the reflective layer instead of the materials described above. However, the pure Ag or alloy comprising pure Cu as the main ingredient involves a drawback that the corrosion resistivity is poor as described above.
Further, pure Al or alloy comprising Al as the main ingredient involves a problem of poor durability. That is, when pure Al or alloy comprising pure Al as the main ingredient is used for the reflective layer of the optical disc, since the disc characteristics tend to cause aging deterioration such as lowering of the reflectivity or increase of errors due to migration or chemical reaction, it is difficult to use the material for the writing-once type optical disc requiring high reliability. Further, the Al material also involves a problem that the reflectivity is low and, particularly, the Al based alloys with addition of alloying elements show further lower reflectivity and it is impossible to attain the high reflectivity of 70% or more in the presence of an organic dye layer.
As described above, while the reflective layer for use in the optical disc is required to satisfy various performances such as high reflectivity, chemical stability (particularly, oxidation resistivity), structural stability (structural stability in the sense that diffusion of Ag is suppressed and growth and cohesion of crystal grains are suppressed) of the recording characteristics and reduced cost, in order to attain a highly reliable media, no metal thin film layer capable of satisfying all the required characteristics has not yet been provided. While Au is most excellent in view of the reflectivity and the chemical stability, it increases the cost and also involves a problem that the reflectivity is lowered greatly for the laser with lower wavelength (405 nm) which will be a standard in the next generation.
This invention has been accomplished in view of the foregoing situations and intends to provide a novel reflective layer or semi-transparent reflective layer for use in optical information recording media, having high reflectivity, as well as being restricted with the crystal grain growth of Ag and excellent in structural stability, optical information recording media and a sputtering target for use in the optical information recording media.
A reflective layer or semi-transparent reflective layer for use in optical information recording media capable of solving the foregoing subject being restricted with the crystal grain growth of Ag according to this invention comprises an Ag based alloy containing:
0.1 to 3.0% of Nd. 
The semi-transparent reflective layer in this invention is a film used as a reflective layer for media conducting multi-layer recording by two or more layers on one side of a disc, and means a thin film having about 20 to 80% of transmittance, the transmittance and the reflectivity being defined depending on the constitution of the disc. Further, the reflective layer in this invention is a thin film used as the reflective layer for single layer recording on one disc surface or as the reflective layer at the lowermost layer for the multi-layer recording, having the transmittance of about 0% and the reflectivity of about 70% or more, being defined depending on the constitution of the disc.
The reflective layer or semi-transparent reflective layer constituted with Agxe2x80x94Nd alloy can suppress diffusion of Ag and suppress the crystal grain growth and, as a result, is excellent in the stability of the crystal structure, which leads to practical effect of the stability of the recording characteristic and, thus, the durability.
In the Agxe2x80x94Nd alloy described above, those further containing from 0.2 to 5.0% in total of at least one element selected from the group consisting of Au, Cu, Pd, Mg, Ti and Ta are preferred embodiments since the oxidation resistivity can be improved. Particularly, an Agxe2x80x94Nd alloy with addition of Au, Cu, Pd (among all, an alloy formed by adding Cu to Agxe2x80x94Nd alloy) is extremely useful which can further improve the oxidation resistivity while suppressing lowering of the reflectivity by alloying.
The reflective layer or semi-transparent reflective layer for use in the optical information recording media according to this invention is applicable to any of the read-only type, writable type or writing-once type disc and, particularly, it can be applied suitably to the writable and writing-once type optical discs.
Further, the optical information recording media having the reflective layer or semi-transparent reflective layer for use in the optical information recording media and the sputtering target for use in the optical information recording media constituted with the Ag based alloy are also included within the range of this invention.